In a conventional wireless communication system, a relay device can be provided in places where radio waves from radio base stations are weak, such as in a building, a tunnel, and a mountain area, thereby enabling signal propagation in such places to be improved. At the relay device, the frequency of a radio wave received from the radio base station and the frequency at which the radio wave is transmitted to a mobile terminal is the same, whereby oscillation may be caused by an interfering signal generated by a transmitted signal that turns back to a receiving antenna. Thus, a relay device has been suggested that can suppress the interfering signal. A relay device has been further suggested that restores the original data from a received signal, regenerates a signal from the restored data, and transmits the regenerated signal (regeneration process), as well as a relay device that transmits the received signal as is (non-regeneration process). As a relay device, a wireless signal booster and/or a repeater described below are also known.
For example, the wireless signal booster includes a first amplifier that amplifies the received signal; a coupler that outputs a cancellation output signal obtained by combining the output from the first amplifier and an input canceller signal; and a quadrature detector for quadrature detection of the cancellation output signal and for conversion into a received quadrature signal. The wireless signal booster further includes a demodulator that demodulates the received quadrature signal and outputs demodulated data; a regenerator that regenerates the quadrature signal, based on the demodulated data and outputs the regenerated quadrature signal; a quadrature modulator for quadrature modulation of the regenerated quadrature signal; and a second amplifier that amplifies the quadrature-modulated signal and outputs a booster output signal. The wireless signal booster further includes a complex correlator that calculates and outputs as a residual interfering signal, the complex function of the quadrature signal received from the quadrature detector and the regenerated quadrature signal from the regenerator; and an attenuator that attenuates the booster output signal from the second amplifier according to attenuation characteristics information that is input. The wireless signal booster further includes a phase shifter that controls the phase of the output from the attenuator according to phase rotation information that is input, and outputs the cancel signal; and a controller that provides, based on the correlation information indicating the level of the residual interfering signal, the attenuation characteristics information to the attenuator and the phase rotation information to the phase shifter. The controller repeatedly changes the attenuation characteristics information and the phase rotation information slightly upward and downward from a given value, detects the correlation information during the change, and updates the attenuation characteristics information and the phase rotation information such that the correlation information decreases within the range of the change (see, for example, Japanese Laid-Open Patent Publication No. H09-284195).
On the other hand, the repeater includes a delay unit that relays a signal from a receiving antenna directly to a transmitting antenna so as to be output therefrom as a transmission signal, and induces a given amount of delay to the received signal; a suppression signal generating unit that generates a suppression signal for suppressing an interfering signal caused by a transmission signal that turns back to the receiving antenna; and a feedback unit that feeds the suppression signal back to the received signal. The suppression signal generating unit detects a residual component by a correlation calculation based on the received signal to which the suppression signal has been fed back and the transmission signal, and generates the suppression signal based on the residual component (see, for example, Japanese Laid-Open Patent Publication No. 2004-40665).
However, the conventional technology has the following problems. In multicast service, multiple radio base stations transmit the same data at the same time to mobile terminals. Thus, a mobile terminal may receive a signal from a radio base station A via a relay device, as well as receive a signal directly from a radio base station B. In this case, delay is caused at the relay device if the relay device performs the regeneration process, and if delay is caused, since the mobile terminal cannot synchronize the signals, the mobile terminal becomes unable to receive the signal transmitted through multicast communication.